This invention relates to vinyl chloride polymers, and specifically to additives which provide smoke suppression to plasticized and rigid polyvinyl chloride ("PVC") polymers without significantly destabilizing such polymers during processing. A "smoke suppressed" PVC polymer composition exhibits "smoke suppression", that is, it exhibits a reduced tendency to generate smoke when it burns, compared to a composition which is not "smoke suppressed".
The excellent mechanical and chemical properties of vinyl chloride homopolymers and copolymers and the versatility of such polymers to compounding variations have resulted in their widespread commercial use for fabricating various structural components, decorative fabrics and films, electrical wiring, floor coverings and the like.
Many commercial applications necessitate the inclusion of substantial quantities, such as about 20 phr to about 100 phr (parts per hundred parts of PVC resin), of ester-type plasticizers in PVC compositions to provide the required degree of flexibility consistent with the intended end use. Other commercial applications of PVC compositions call for the PVC resin to be used in rigid, i.e. unplasticized, form.
Burning of a PVC polymer composition can cause the PVC to degrade, giving off soot and suspended particles which form what is generally termed "smoke". The degradation products can also include aromatic and aliphatic hydrocarbons, carbon oxides, and hydrochloric acid. In the case of plasticized PVC, the compounds forming the smoke can include not only the hydrocarbons, carbon oxides, and HCl from the PVC, but also the plasticizer compounds and degradation products thereof. When the PVC composition is exposed to high temperatures accompanying a fire, the tendency of the PVC to give off smoke can create a source of danger to imperiled personnel which can be greater than the fire itself.
Thus, it is desirable to incorporate into PVC polymer compositions a smoke suppressant, that is, a compound which will inhibit the formation of smoke when the composition burns. A variety of organic and inorganic compounds and salts have heretofore been proposed or used to reduce the smoke generation characteristics of rigid or plasticized PVC polymer compositions, but such agents have had drawbacks such as not providing an optimum improvement in smoke suppression for both rigid and plasticized PVC compositions, and unduly reducing the stability or processability of the polymer composition.
Because of the undesirable effects on the PVC of the elevated temperatures encountered during compounding and fabricating of PVC compositions, it is customary to incorporate into the PVC composition one or more heat stabilizers. The heat stabilizers decrease the vulnerability of the PVC to degradation, and stabilize the PVC against other unwanted thermally induced effects, such as changes in polymer structure, viscosity, and density. However, some agents employed to impart smoke suppression to polyvinyl chloride compositions unduly decrease the heat stability of the polymer compositions in which they are incorporated. In particular, some commercial smoke suppressant additives based on zinc compounds or combinations of zinc compounds with other compounds contain free zinc oxide, which can accelerate the degradation of polyvinyl chloride resins on exposure to temperatures above about 100.degree. C. There is thus a need for a smoke suppressant additive which can be incorporated into a heat stabilized polyvinyl chloride formulation without destabilizing the formulation, that is, without significantly detracting from the heat stability of the polyvinyl chloride composition.
A material which is effective as a smoke suppressant for PVC polymers, and which has recently become available, comprises a talc-extended composition based on Zn.sub.3 Mo.sub.2 O.sub.9.H.sub.2 O. The preparation and properties of this material are shown in greater detail in the examples accompanying this application. As for other smoke suppressants, U.S. Pat. No. 4,098,748 and U.S. Pat. No. 4,098,753 disclose that smoke-suppressant and flame-resistant properties may be imparted to plasticized polyvinyl chloride compositions by additions of about 1 to about 8 phr of MoO.sub.2, MoO.sub.3, (NH.sub.4).sub.2 Mo.sub.2 O.sub.7, (NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, (NH.sub.4).sub.4 Mo.sub.8 O.sub.26, (NH.sub.4).sub.2 Mo.sub.10 O.sub.31, Na.sub.2 MoO.sub.4, CaMoO.sub.4, or ZnMoO.sub.4. U.S. Pat. No. 3,900,441 discloses that "zinc molybdate" of unspecified molecular structure, or alternatively physical mixtures of zinc compounds and molybdenum compounds, can be used as smoke suppressants for polyvinyl chloride-containing compositions. While these smoke suppressants are useful in many applications, they do not comprise or suggest the particular synergistically effective smoke suppressant additive which is incorporated into PVC formulations in accordance with the present invention.
The present invention overcomes many of the problems and disadvantages associated with vinyl chloride polymers by providing a resin formulation including effective amounts of a material which functions as a smoke suppressant agent without detracting from the heat stability and processability of the polymer; which is of comparatively low toxicity; which is not itself volatilized when heated to an elevated pyrolysis temperature, but instead remains in the char; which is more economical than many other agents heretofore employed on a cost-effectiveness basis; which is available in good supply from domestic sources; and which is of versatile use in compounding vinyl chloride polymeric compositions, preferably contributing a white color to the polymer composition to allow coloring and coding of wire and cables.